David Boies, the star trial lawyer who helped lead the legal charge that overturned California’s same-sex marriage ban, is becoming chairman of the Partnership for Educational Justice, a group that former CNN anchor Campbell Brown founded in part to pursue lawsuits challenging teacher tenure.

Mr. Boies, the son of two public schoolteachers, is a lifelong liberal who represented Al Gore in Bush v. Gore and prosecuted Microsoft in the Clinton Administration’s antitrust suit. In aligning himself with a cause that is bitterly opposed by teachers’ unions, he is emblematic of an increasingly fractured relationship between the Democrats and the teachers’ unions.